摘要:

The mechanical response of polycrystalline materials, particularly under shock loading, is of significant interest in a variety of munitions and industrial applications. Homogeneous continuum models have been developed to describe material response, including Equation of State, strength, and reactive burn models. These models provide good estimates of bulk material response. However, there is little connection to underlying physics and, consequently, they cannot be applied far from their calibrated regime with confidence. Both explosives and metals have important structure at the (energetic or single crystal) grain scale. The anisotropic properties of the individual grains and the presence of interfaces result in the localization of energy during deformation. In explosives energy localization can lead to initiation under weak shock loading, and in metals to material ejecta under strong shock loading. To develop accurate, quantitative and predictive models it is imperative to develop a sound physical understanding of the grain‐scale material response.Numerical simulations are performed to gain insight into grain‐scale material response. The Generalized Interpolation Material Point Method family of numerical algorithms, selected for their robust treatment of large deformation problems and convenient framework for implementing material interface models, are reviewed. A three‐dimensional simulation of wave propagation through a granular material indicates the scale and complexity of a representative grain‐scale computation. Verification and validation calculations on model bimaterial systems indicate the minimum numerical algorithm complexity required for accurate simulation of wave propagation across material interfaces and demonstrate the importance of interfacial decohesion. Preliminary results are presented which predict energy localization at the grain boundary in a metallic bicrystal. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780213

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

The simulation of high‐velocity impact and penetration is inhibited by complex material behavior and large deformations. Lagrangian formulations best model complex materials because history‐dependent variables and material boundaries are not advected. However, Lagrangian finite elements are limited by large deformations. Recently, meshfree particle methods have been used to avoid such limitations, and have demonstrated greater accuracy than traditional erosion methods (wherein deformed elements are removed). Though the variable connectivity of particles enables them to model large deformations, it requires more computational effort than (fixed‐connectivity) elements. Therefore, an algorithm was designed to convert deformed elements to particles, thus providing the ability to model large deformations where needed, while maintaining the efficiency of elements elsewhere. This combination is essential in three dimensions, where problem size demands efficiency. In this paper, the conversion algorithm is demonstrated for several three‐dimensional simulations of high‐velocity impact and penetration. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780214

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Three dimensional phononic crystal (“sonic vacuum” without prestress) was assembled from 137 vertical cavities arranged in hexagonal pattern in Silicone matrix filled with stainless steel spheres. This system has unique strongly nonlinear properties with respect to wave propagation inherited from nonlinear Hertz type elastic contact interaction. Trains of strongly nonlinear solitary waves excited by short duration impact were investigated. Solitary wave with speed below sound speed in the air and reflection from the boundary of two “sonic vacuums” were detected. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780215

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

The wave propagation phenomenon in a single straight chain of disks made of PSM9 was simulated numerically using the finite element code ABAQUS. The results yield information on the stress wave propagation along the chain. Qualitative agreement with experimental data that was obtained using an optical method of dynamic photo‐elasticity and strain gages was obtained. The results of the comparison clearly demonstrate that the stress propagation phenomena are largely governed by the quality of the contacts between the disks. Based on the obtained results justifications for further research efforts on this subject are presented. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780216

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

The present effort examines shock‐wave propagation in porous ceramic powders. Computational models of varying sophistication have been developed to treat the dynamic compaction of porous media. The preponderance of computational treatments in production codes, however, have used relatively straightforward engineering modeling approaches to the stress‐wave induced compaction of porous matter such as the Herrmann p‐alpha model, and a more recent method identified as the p‐lambda model. Analytic solutions of shock propagation in porous media have also been fruitfully pursued as exemplified by the seminal solutions of Kompaneets outlined in the second volume of the Zeldovich and Raiser treatise on shock wave physics. Analytic solutions offer instructive insight into the phenomena of shock propagation in porous media and allow scaling of the governing equations to identify the prevailing material and boundary properties. Here the solution methods of Kompaneets have been extended to include specific compaction models. Relationships between compaction models and the resulting shock propagation are explored. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780217

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Both Ti and Zr exhibit phase transitions from the &agr; (hcp) to the &ohgr; phase at pressures of a few GPa. In addition, the Hugoniot of Zr shows a second phase transition at 23 GPa. We have developed multi‐phase equations of state for these metals based on ultrasonic, static compression, and shock data. The second transition in Zr is consistent with a phase diagram in which the high‐temperature and high‐pressure bcc phases are a single continuous phase. Time‐resolved experiments using plate impact and continuous magnetic loading are compared to simulations to investigate the kinetics of these phase transitions. Strong kinetic effects are observed in the &agr; ‐ &ohgr; transition in both metals, with the dynamic phase transition observed at pressures well above the equilibrium phase boundary. Data on Zr samples of varied purity are consistent with a strong reduction of the transformation rate by impurities. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780218

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

As part of a larger project into the interactions of shock with grain boundaries, calculations that show a localization of energy in some configurations were performed. The verification and prediction of these localizations become important, due to their role in the initiation of a variety of significant material process. The prototype problem is a single grain boundary, inclined to the direction of shock propagation, separating regions of differing orientation. The calculations are made with a finite volume code using a continuum material model with explicit elastic, and plastic anisotropy. The response of NiAl is simulated using material property data from published sources, and from new experiments performed as part of the over‐all project. The localization is seen internally as a small region of higher pressure at the intersection of the shock and the grain boundary. At the breakout surface the localization becomes manifest in velocity and displacement excursions at the grain boundary. This surface phenomenon provides an observable that can be used, with planned experiments, to validate the predictive behavior of the model. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780219

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Techniques for obtaining continuously distributed local properties — such as density, velocity, pressure, and temperature — from atomistic simulations are discussed. The resulting local properties are averages over nanometer sized circular regions and are defined so that the continuum expressions for mass, momentum, and energy conservation are exactly satisfied. The techniques are illustrated by calculating the two dimensional spatial distribution of local temperature that results from a shock wave passing over a void. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780220

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Molecular dynamics simulation on a fcc perfect crystal with the Lennard‐Jones potential is performed in order to investigate temperature dependence of shock‐induced plasticity. It is found that the critical piston velocity above which stacking faults emerge shifts downwards once the temperature exceeds approximately half the melting temperature. Also Hugoniot elastic limit is found to be a decreasing function of temperature, whereas the corresponding critical strain is insensitive to temperature. The discrepancy between the simulation and the experiments where Hugoniot elastic limit is a increasing function of temperature is discussed. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780221

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Shocks are often simulated using the classical molecular dynamics (MD) method in which the electrons are not included explicitly and the interatomic interaction is described by an effective potential. As a result, the fast electronic heat conduction in metals and the coupling between the lattice vibrations and the electronic degrees of freedom can not be represented. Under conditions of steep temperature gradients that can form near the shock front, however, the electronic heat conduction can play an important part in redistribution of the thermal energy in the shocked target. We present the first atomistic simulation of a shock propagation including the electronic heat conduction and electron‐phonon coupling. The computational model is based on the two‐temperature model (TTM) that describes the time evolution of the lattice and electron temperatures by two coupled non‐linear differential equations. In the combined TTM‐MD method, MD substitutes the TTM equation for the lattice temperature. Simulations are performed with both MD and TTM‐MD models for an EAM Al target shocked at 300 kbar. The target includes a tilt grain boundary, which provides a region where shock heating is more pronounced and, therefore, the effect of the electronic heat conduction is expected to be more important. We find that the differences between the predictions of the MD and TTM‐MD simulations are significantly smaller as compared to the hydrodynamics calculations performed at similar conditions with and without electronic heat conduction. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780222

出版商:AIP    

年代:1904

数据来源: AIP